INTERNET-DRAFT D. Meyer
draft-ietf-grow-collection-communities-06.txt
Category Best Current Practice
Expires: March 2005 September 2004
BGP Communities for Data Collection<draft-ietf-grow-collection-communities-06.txt>
Status of this Memo
Status of this Memo
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D. Meyer [Page 1]

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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
BGP communities (RFC 1997) are used by service providers for many
purposes, including tagging of customer, peer, and geographically
originated routes. Such tagging is typically used to control the
scope of redistribution of routes within a provider's network, and to
its peers and customers. With the advent of large scale BGP data
collection (and associated research), it has become clear that the
information carried in such communities is essential for a deeper
understanding of the global routing system. This memo defines
standard (outbound) communities and their encodings for export to BGP
route collectors.
D. Meyer [Page 2]

INTERNET-DRAFT Expires: March 2005 September 20041. Introduction
BGP communities [RFC1997] are used by service providers for many
purposes, including tagging of customer, peer, and geographically
originated routes. Such tagging is typically used to control the
scope of redistribution of routes within a providers network, and to
its customers and peers. Communities are also used for a wide variety
of other applications, such as allowing customers to set attributes
such as LOCAL_PREF [RFC1771] by sending appropriate communities to
their service provider. Other applications include signaling various
types of VPNs (e.g., VPLS [VPLS]), and carrying link bandwidth for
traffic engineering applications [EXTCOMM].
With the advent of large scale BGP data collection [RIS,ROUTEVIEWS]
(and associated research), it has become clear that the geographical
and topological information, as well as the relationship the provider
has to the source of a route (e.g., transit, peer, or customer),
carried in such communities is essential for a deeper understanding
of the global routing system. This memo defines standard communities
for export to BGP route collectors. These communities represent a
significant part of information carried by service providers as of
this writing, and as such could be useful for internal use by service
providers. However, such use is beyond the scope of this memo.
Finally, those involved in BGP data analysis are encouraged to verify
with their data sources as to which peers implement this scheme (as
there is a large amount of existing data as well as many legacy
peerings).
The remainder of this memo is organized as follows. Section 2
provides both the definition of terms used as well as the semantics
of the communities used for BGP data collection, and section 3
defines the corresponding encodings for RFC 1997 [RFC1997]
communities. Finally, section 4 defines the encodings for use with
extended communities [EXTCOMM].
2. Definitions
In this section, we define the terms used and the categories of
routes that may be tagged with communities. This tagging is often
refered to as coloring, and we refer to a route's "color" as its
community value. The categories defined here are loosely modeled on
those described in [WANG] and [HUSTON].
D. Meyer Section 2. [Page 4]

INTERNET-DRAFT Expires: March 2005 September 20042.1. Peers and Peering
Consider two network service providers, A and B. Service providers A
and B are defined to be peers when (i). A and B exchange routes via
BGP, and (ii). traffic exchange between A and B is settlement-free.
This arrangement is also typically known as "peering". Peers
typically exchange only their respective customer routes (see
"Customer Routes" below), and hence exchange only their respective
customer traffic. See [HUSTON] for a more in-depth discussion of the
business models surrounding peers and peering.
2.2. Customer Routes
Customer routes are those routes which are heard from a customer via
BGP and are propagated to peers and other customers. Note that a
customer can be an enterprise or another network service provider.
These routes are sometimes called client routes [HUSTON].
2.3. Peer Routes
Peer routes are those routes heard from peers via BGP, and not
propagated to other peers. In particular, these routes are only
propagated to the service provider's customers.
2.4. Internal Routes
Internal routes are those routes that a service provider originates
and passes to its peers and customers. These routes are frequently
taken out of the address space allocated to a provider.
2.5. Internal More Specific Routes
Internal more-specific routes are those routes which are frequently
used for circuit load balancing purposes, IGP route reduction, and
also may correspond to customer services which are not visible
D. Meyer Section 2.5. [Page 5]

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outside the service provider's network. Internal more specific routes
are not exported to any external peer.
2.6. Special Purpose Routes
Special purpose routes are those routes which do not fall into any of
the other classes described here. In those cases in which such routes
need to be distinguished, a service provider may color such routes
with a unique value. Examples of special purpose routes include
anycast routes, and routes for overlay networks.
2.7. Upstream Routes
Upstream routes are typically learned from upstream service provider
as part of a transit service contract executed with the upstream
provider.
2.8. National Routes
These are route sets that are sourced from and/or received within a
particular country.
2.9. Regional Routes
Several global backbones implement regional policy based on their
deployed footprint, and on strategic and business imperatives.
Service providers often have settlement-free interconnections with an
AS in one region, and that same AS is a customer in another region.
This mandates use of regional routing, including community attributes
set by the network in question to allow easy discrimination among
regional routes. For example, service providers may treat a route set
received from another service provider in Europe differently than the
same route set received in North America, as it is common practice to
sell transit in one region while peering in the other.
D. Meyer Section 2.9. [Page 6]

INTERNET-DRAFT Expires: March 2005 September 20043. RFC 1997 Community Encoding and Values
In this section we provide RFC 1997 [RFC1997] community values for
the categories described above. RFC 1997 communities are encoded as
BGP Type Code 8, and are treated as 32 bit values ranging from
0x0000000 through 0xFFFFFFF. The values 0x0000000 through 0x0000FFFF
and 0xFFFF0000 through 0xFFFFFFFF are reserved.
The best current practice among service providers is to use the high
order two octets to represent the provider's AS number, and the low
order two octets to represent the classification of the route, as
depicted below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <AS> | <Value> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where <AS> is the 16 bit AS number. For example, the encoding
0x2A7C029A would represent the AS 10876 with value 666.
3.1. Community Values for BGP Data Collection
In this section we define the RFC 1997 community encoding for the
route types described above for use in BGP data collection. It is
anticipated that a service provider's internal community values will
be converted to these standard values for output to a route
collector.
This memo follows the best current practice of using the basic format
<AS>:<Value>. The values for the route categories are described in
the following table:
D. Meyer Section 3.1. [Page 7]

INTERNET-DRAFT Expires: March 2005 September 20044.1. Four-octet AS specific extended communities
The four-octet AS specific extended community is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | 0x05 | Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Administrator (cont.) | 0x10F2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this case, the 4 octet Global Administrator sub-field contains a
4-octets Autonomous System number assigned by the IANA.
5. Note on BGP Update Packing
Note that data collection communities have the potential of making
the attribute set of a specific route more unique than it would be
otherwise (since each route collects data that is specific to it's
path inside one or more ASes). This, in turn, can affect whether
multiple routes can be grouped in the same BGP update message, and
may lead to increased use of bandwidth, router CPU cycles, and
memory.
6. Acknowledgments
The community encoding described in this memo germinated from an
interesting suggestion from Akira Kato at WIDE. In particular, the
idea would be to use the collection community values to select paths
that would result in (hopefully) more efficient access to various
services. For example, in the case of RFC 3258 [RFC3258] based DNS
anycast service, BGP routers may see multiple paths to the same
prefix, and others might be coming from the same origin with
different paths, but others might be from different region/country
(with the same origin AS).
Joe Abley, Randy Bush, Sean Donelan, Xenofontas Dimitropoulos, Vijay
Gill, John Heasley, Geoff Huston, Steve Huter, Michael Patton,
Olivier Marce, Ryan McDowell, Rob Rockell, Rob Thomas, Pekka Savola,
D. Meyer Section 6. [Page 11]

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Patrick Verkaik and Alex Zinin all made many insightful comments on
early versions of this draft. Henk Uijterwaal suggested the use of
the ISO-3166-2 country codes.
D. Meyer Section 6. [Page 12]

INTERNET-DRAFT Expires: March 2005 September 20047. Security Considerations
While this memo introduces no additional security considerations into
the BGP protocol, the information contained in the communities
defined in this memo may in some cases reveal network structure that
was not previously visible outside the provider's network. As a
result, care should be taken when exporting such communities to route
collectors. Finally, routes exported to a route collector should also
be tagged with the NO_EXPORT community (0xFFFFFF01).
7.1. Total Path Attribute Length
The communities described in this memo are intended for use on egress
to a route collector. Hence an operator may choose to overwrite its
internal communities with the values specified in this memo when
exporting routes to a route collector. However, operators should in
general ensure that the behavior of their BGP implementation is well-
defined when the addition of an attribute causes a PDU to exceed 4096
octets. For example, since it is common practice to use community
attributes to implement policy (among other functionality such as
allowing customers to set attributes such as LOCAL_PREF), the
behavior of an implementation when the attribute space overflows is
crucial. Among other behaviors, an implementation might usurp the
intended attribute data or otherwise cause indeterminate failures.
These behaviors can result in unanticipated community attribute sets,
and hence result in unintended policy implications.
8. IANA Considerations
This memo assigns a new Sub-Type for the AS specific extended
community type. In particular, the IANA should assign Sub-type 0x05,
using the "First Come First Served" policy defined in RFC 2434
[RFC2434], for the Sub-Type defined in Section 4. This corresponds to
a Type Field value of 0x0005.
D. Meyer Section 8. [Page 13]

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[VPLS] Kompella, K., et al., "Virtual Private LAN
Service", draft-ietf-l2vpn-vpls-bgp-02.txt,
Work in Progress.
[WANG] Wang, F. and L. Gao, "Inferring and Characterizing
Internet Routing Policies", ACM SIGCOMM Internet
Measurement Conference 2003.
10. Author's Addresses
David Meyer
EMail: dmm@1-4-5.net
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D. Meyer Section 10. [Page 16]